Installations for the treatment of sewage are generally known. The aim with the conventional installations is to lower the content of organic components and nitrogen in the effluent.
However, with conventional installations of this type the phosphorus content remains virtually unchanged.
A drastic reduction in the discharge of the so-called priority substances, which also include nitrogen and phosphate, is desirable. New sewage treatment plants (STPs) have to be developed for this drastic removal of nitrogen and phosphate.
To date, STPs have been built which partially treat the (raw) sewage with the aid of physical, biological and/or chemical techniques. A sewage treatment plant of this type roughly consists of the following components:
(1) a coarse filter; PA1 (2) a sand trap; PA1 (3) a pre-settling tank; PA1 (4) a (partially) aerated basin; PA1 (5) a post-settling tank; PA1 (6) a sludge thickener; and PA1 (7) if desired, a sludge, digester and/or sludge dewatering. PA1 a) in a first step phosphate is removed by biological means and, at the same time, the chemical and biological oxygen demand (COD and BOD) are reduced in a highly loaded active sludge system, PA1 b) in a second step, a nitrification is carried out, ammonium being converted to nitrate, and PA1 c) in a third step, a denitrification is carried out using a carbon source, in which method the order of the second and the third step is exchangeable. PA1 1) Biological phosphorus removal and simultaneous reduction of COD, PA1 2) denitrification, preferably in a biofilm reactor and PA1 3) nitrification, preferably in a biofilm reactor.
In steps 1, 2 and 3, particles are removed from (raw) sewage with the aid of physical techniques. In step 4 biodegradable organic components are consumed by micro-organisms. In addition to heterotrophic bacteria, which convert biodegradable organic material, the mixed liquor of low-loaded systems (e.g. an oxidation ditch or carrousel) also contains autotrophic nitrifying bacteria, which convert ammonia to nitrate.
Sometimes the mixed liquor of low loaded sewage treatment plants contains also denitrifying microorganisms, which convert the nitrate formed into harmless nitrogen gas. The denitrification in STPs is often incomplete, with the consequence that the effluent contains relatively large amounts of nitrate.
The sludge of highly loaded STPs contains no nitrifying microorganisms as a result of which nitrification does not take place. As a consequence the effluent from high loaded STPs contains a large amount of ammonium nitrogen.
Only about 30% of the phosphate in the raw sewage is removed in the existing sewage treatment plants with the consequence that the effluent contains a relatively high concentration of phosphate.